Grippers are known capable of being dispaced along an elongated support and of locking itself in at least one direction of displacement at any location along the elongated body, by lockage of two jaws provided with surfaces for application on the body. The lockage is obtained by wedge effect, each jaw being associated with a pair of bearing surfaces inclined in the same direction with respect to the elongated body, one being on the jaw, the other on a rigid body fixed to the gripper. The assembly is mounted with play which permits the displacement of the gripper along the elongated body.
Generally, the displacement is possible in one direction, the displacement in the other direction producing a self-lockage of the jaws. In this regard, a rigid reaction body externally surrounds the jaws and includes two lateral sides on which are provided the inclined bearing faces and which are capable of resisting a separation force opposite the force for locking the jaws. When the gripper is displaced in relation to the elongated body in the direction of convergence of the inclined bearing surfaces, the play is eliminated and the inclined bearing surfaces form with the surfaces for application of the jaws on the elongated body a wedge which blocks the displacement of the gripper. The suppression of the play can be obtained by longitudinal displacement of one of the bearing surfaces of each pair with respect to the other by means of springs or jacks.
Of course, the displacements are relative, the elongated body being able to be displaced with respect to the gripper or vice versa. In the first case, the rigid reaction body is fixed, the gripper bearing thereon. However, the elongated body can itself be fixed, the force of displacement of the gripper then being applied to the rigid body. In both cases, the longitudinal force which the gripper is required to resist is transformed into a transverse locking force which maintains the gripper in position by friction. This force is proportional to the applied force, the factor of proportionality depending on the angle of inclination of the surfaces. It is sufficient if the resulting friction force of the jaws on the support is greater than the applied force on the gripper to obtain the blockage of the support at any point on the support whatever the load.
Such grippers are often composed of separate elements, the jaws being wedges placed between the exterior body and the support and force-fit. The unlocking can, in this case, cause difficulties.
Furthermore, it is not easy to determine at what moment, the play being taken up, the transverse locking force is integrally transmitted by the rollers and there is a risk of loss by friction.
There has also been proposed the interposition between the inclined bearing faces of members rolling on the two faces, such as cylindrical rollers or balls, which permit displacement of the jaws with respect to the exterior body in a manner to take up the play and which transmit the locking forces. This arrangement diminishes the risk of lockage but the construction of the gripper is often very complicated. On the other hand, when the gripper is opened it is not certain that the rollers return to their correct place. Additionally, the rollers can slide and flats can be formed on their surface.